IPS is iron pipe size. CTS is copper tube size. Some materials are better for a given application, for one reason or another. But if you want to, and there are no other overriding factors, you can exchange one type, IPS for example, for CTS copper tubing. Some might question why you would do it, but if that were your preference there would be no regulation against it.

: What is IPS and CTS? Why the different use for different piping applications?